Remove OpenJDK-alpine Dockerfile template

What does this MR do?

Removes OpenJDK-alpine Dockerfile template since the upstream dropped to support alpine-based openjdk 2 years ago (2019-03-27 UTC). See deprecation of alpine-based openjdk at https://github.com/docker-library/openjdk/pull/322.

The deleted file was initially introduced in gitlab-foss!11158 (merged) by @timothyandrew for %9.2.

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?) database related changes?](https://docs.gitlab.com/ee/development/database_review.html))
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Closes #324615 (closed)